Method of making alloy steel powder

ABSTRACT

AN ALLOY STEEL POWDER TO BE USED FOR POWDER METALLURGY PROCESSES. THE POWDER IS PRODUCED BY ATOMIZING A MOLTEN STREAM OF ALLOY STEEL CONTAINING UP TO 0.40% BY WEIGHT OF CARBON AND CONTAINING ONE OR MORE OF THE FOLLOWING ELEMENTS:   PERCENT NICKEL 0.20 TO 3.0 MOLYBDENUM 0.2 TO 1.0 CHROMIUM 0.2 TO 1.0   THE SILICON AND MANGANESE CONTENTS OF THE STEEL SHOULD BE LESS THAN 0.10% AND .50% BY WEIGHT RESPECTIVELY. FOLLOWING THE ATOMIZATION, THE RESULTANT PARTICLES ARE HEAT TREATED AT A TEMPERATURE OF 1500-2100*F. TO SOFTEN THE STEEL AS WELL AS REDUCING THE CARBON CONTENT. AFTER ANNEALING, THE CAKE-LIKE STRUCTURE IS BROKEN UP BY HAMMER-MILLING TO RESTORE THE AS-ATOMIZED PARTICLE SIZE.

United States Patent US. Cl. 75-.5 BA 5 Claims ABSTRACT OF THE DISCLOSURE An alloy steel powder to be used for powder metallurgy processes. The powder is produced by atomizing a molten stream of alloy steel containing up to 0.40% by weight of carbon and containing one or more of the following element's:

Percent Nickel 0.20 to 3.0 Molybdenum 0.2 to 1.0 Chromium 0.2 to 1.0

The silicon and manganese contents of the steel should be less than 0.10% and .50% by weight respectively.

Following the atomization, the resultant particles are heat treated at a temperature of 1500-2100 F. to soften the steel as well as reducing the carbon content. After annealing, the cake-like structure is broken up by hammer-milling to restore the as-atomized particle size.

This application is a continuation-in-part of application Ser. No. 740,070 filed June 26, 1968, and now abandoned.

The invention relates to an alloy steel powder to be used in powder metallurgy processes and to a method of forming the powder.

There are several basic procedures by which metal powder to be used in powder metallurgy processes can be prepared. The metal powder can be prepared by electrolytic processes, reduction processes or by air or by water atomization processes, such as that described in the United States Patent 3,325,277 of Robert A. Huseby. According to the process of that patent, molten steel is fed by gravity in the form of a downwardly moving stream and a series of flat sheets of water are impinged against the stream of molten steel at an angle to thereby atomize the molten stream and produce a plurality of agglomerates of spheroidal particles. Subsequently the particles are annealed at a temperature of about 1500-l800 F. in a reducing atmosphere for a period of time sutficient to soften the particles and reduce the carbon content to a value of less than 0.05%. Following the annealing treatment the particles are subjected to hammer-milling to break up the cake-like structure formed during the anneal and restore the as-atomized particle size.

The steel powder formed according to the method of Patent 3,325,277 when compacted and sintered, has a high density and superior physical properties.

It has been found that when using a process such as that set forth in Patent 3,325,277 to form alloy steel powder utilizing conventional alloy steel formulations problems arise due to the low compressibility of the alloy steel powder. Conventional heat treatable alloy steels generally contain a combination of several of the following elements; from 0.40 to 3.75% nickel, 0.30 to 1.6% chromium, 0.08 to 0.60% molybdenum, 0.70% to 0.90% manganese and 0.20 to 1.6% silicon. The above alloying elements form oxides during atomization and the subsequent annealing treatment, and it has been found that the oxides of silicon and manganese are extremely refractive and are difficult to reduce during the anneal. This results in the powder formed from conventional alloy steel compositions having a high oxide content in the form of oxide inclusions which reduces the ductility, impact strength and fatigue strength of the resulting compacted powder.

In addition, the unreduced oxides of manganese and silicon increase the hardness of the individual steel particles thereby providing the powder with a low compressibility.

The present invention is directed to an improved alloy steel powder having a reduced oxygen content or hydrogen loss and having a high compressibility.

The steel to be used in the process of the invention can be produced by one of the conventional steel making processes such as open hearth, electric furnace, basic oxygen and the like. The steel contains up to 0.40% by weight of carbon and preferably from 0.06% to 0.12% carbon. In addition the alloy steel contains one or more of the following elements 0.20 to 3.0% nickel, 0.20 to 1.0% chromium and 0.20 to 1.0% molybdenum.

The silicon content of the alloy steel should be maintained less than 0.10% by weight and in the range of 0.01 to 0.10%, while the manganese content should preferably be less than 0.30% by weight, and in the range of 0.05 to 0.30%, but can be as high as 0.50% by weight when the alloy contains substantial additions of chromium, nickel and/ or molybdenum.

In addition, the titanium and aluminum contents of the alloy should be less than 0.05% and sulfur and phosphorus should be less than 0.04% and 0.035% respectively.

The steel powder is produced by an apparatus similar to that shown in Patent 3,325,277 of Huseby. The molten steel in the tundish is at a temperature of about 3100 F. and flows by gravity from the tundish through a series of outlet slots or nozzles. A thin sheet or curtain of water is directed against the streams of molten steel at an angle greater than 5 with respect to the axis of the stream and generally at an angle of 15 to 55 from the vertical.

The temperature of the water employed in the atomization process is not critical and is generally less than 160 F. The water is under substantial pressure, generally above 500 p.s.i., and for most operations above 1000 p.s.i. There is no maximum pressure limit for the water and normally the maximum pressure is based on the pumping equipment used. In the atomization the water pressure is correlated with the angle at which the water sheets are directed against the molten metal stream. As the angle is decreased and approaches the vertical, the water pressure must correspondingly increase. Generally, the horizontal component of water velocity should be above feet per second to produce the desired agglomerated type of particles.

The water is preferably in the form of thin sheets having a thickness less than 0.075 inch and preferably less than 0.05 inch at the point of discharge from the nozzle. The nozzles are designed with respect to the molten streams so that the sheets of water do not flair out to any appreciable extent but maintain the thickness when impinging against the molten steel stream.

The thin sheets of water strike the molten steel stream and atomize or particalize the steel to produce chainlike agglomerates of generally spheroidal particles. The steel powder as-atomized has a particle size such that at least 85% will pass through an 80 mesh sieve and at least 75% will pass through a 100 mesh sieve.

Following the atomization, the steel powder is subjected to an annealing treatment which serves to soften the particles, reduce the oxide film and substantially decrease the carbon content. During the anneal the powder is heated to a temperature in the range of 1500 F. to 2100" F. and preferably 1650 F. to 1850 F. in a reducing atmosphere such as disassociated ammonia, hydrogen or other conventional decarburizing reducing gases.

During the annealing, the carbon content of the steel particles is reduced to a value below 0.05% and generally to a value in the range of 0.001 to 0.02%. The annealed powder has an oxygen content less than 0.40% and in most cases in the range of 0.01% to 0.25%. If chromium is not used in the alloy steel, or if the chromium content is in the lower portion of the aforementioned range, the oxygen content will generally be below 0.25%. If the chromium content is in the upper portion of the aforementioned range, the oxygen content may be above 0.25% and below 0.40%. To obtain the optimum ductility and subsequently obtain the maximum density for a given compaction pressure, and increased physical properties in the sintered product, the powder should be maintained at the annealing temperature for a period of at least 1V2 hours and preferably 2 hours.

Following the anneal, the particles are generally caked together and are broken apart by a hammer-milling processs. The hammer-milling, which is an impact process, breaks up the sintered cake while not breaking up the irregular agglomerated nature of the particles and serves to restore the as-atomized particle size.

The resulting alloy steel powder has an apparent density which is a non-compacted density, as defined by test procedure ASTM B21248, in the range of 2.6 to 3.3 grams/cc. The alloy steel powder has a pressed density of over 6.4 grams/cc. and generally in the range of 6.4 to 6.8 grams/cc. The pressed density is based on a compaction pressure of 30 tons per square inch as defined in the test procedure ASTM B-33 1-5 8T, except that 0.5% dry zinc stearate lubricant was mixed with the powder.

The resulting alloy steel powder can be used to form various machine parts or combinations of parts by conventional powder metallurgy procedures. A conventional lubricant, such as zinc stearate, and additional carbon, if desired, can be blended with the alloy steel powder by suitable blending equipment. The blended powder is then compacted into the desired shape by a compaction pressure generally above 15 tons per square inch and preferably at 30 tons per square inch or more.

Following the compaction, the alloy steel powder is then sintered in a reducing atmosphere at a temperature in the range of 2000 F. to 2300 F. for a period of 10 minutes to 1 hour depending on the composition and the final density desired. The sintered parts are suitable for secondary heat treatment, such as carburizing, carbonitriding, nitriding, or straight heat treatment including quenching and tempering. Furthermore, the sintered parts can be recompressed after sintering either by hot or cold, to obtain a density approaching the 100% theoretical density of the metal.

The particle size of the steel powder is determined by the atomization step and no grinding, crushing or milling operation is required to obtain a small particle size as is necessary in most conventional processes. The process of hammer-milling after annealing acts to break up the sintered cake and is not a true grinding operation for the size of the individual particles is not reduced but the particle size is merely restored to the atomized condition.

Specific examples of the process of the invention are as follows:

EXAMPLE 1 Molten steel having the following composition in weight percent was supplied to a tundish.

The temperature of the molten alloy steel was 3050 F. and the steel flowed downwardly by gravity through an outlet nozzle having an internal diameter of 5 inch. Two oppositely directed streams or curtains of water positioned at a downward angle of 33 with respect to the axis of the molten steel stream were impinged against the molten metal to atomize the alloy steel. The temperature of the water was initially 68 F. and had a final temperature of 138 F. The water was under pressure of 1050 p.s.i. and had a flow rate of 860 gallons per minute. The water streams were discharged through slots 3 inches long and 0.04 inch wide.

The resulting as-atomized alloy steel powder had the following screen analysis.

Percent retained Tyler sieve size: on sieve 48 Trace Pan 49.3

The alloy steel powder was then annealed in disassociated amonia at a temperature of 1700 F. for 2 hours, subsequently cooled in a dissociated ammonia atmosphere to F. and then air cooled to room temperature.

The annealed steel powder had the following analysis in weight percent:

The alloy steel powder was then broken up by hammermilling to the as-atomized state and had an apparent density of 2.85 grams/co, a pressed density at a compaction pressure of 30 tons per square inch of 6.45 grams/ cc. and a green strength of 1100 p.s.i., with a .75 zinc stearate lubricant, after pressing at 30 tons per square inch.

After annealing and hammer-milling the alloy steel powder had the following sieve analysis.

Percent retained Tyler sieve size: on sieve 80 .6

Pan 29.6

EXAMPLE 2 Molten steel having the following composition in weight percent was supplied to a tundish:

Carbon .140

Manganese .120 Phosphorus .011 Sulphur .015 Silicon .110

Chromium .270 Nickel .280 Molybdenum .210 Iron Balance The steel was atomized and annealed by the procedure set forth in Example 1. Following the anneal the sintered cake was broken up by hammer-milling.

The annealed steel powder had the following analysis in weight percent.

After breaking up by hammer-milling, the alloy steel powder had an apparent density of 2.80 grams/cc., a pressed density at a compaction pressure of 30 tons per square inch of 6.60 grams/cc. and a green strength of 1150 p.s.i., with .75 zine stearate lubricant, and after pressing at 30 tons per square inch.

EXAMPLE 3 Molten steel having the following composition in weight percent was supplied to a tundish:

Carbon .100 Manganese .150 Phosphorus .010 Sulphur .017 Silicon .007 Nickel 1.760 Molybdenum .430 Iron Balance The molten steel was atomized, annealed and hammermilled according to the procedure outlined in Example 1. The annealed and hammer-milled alloy steel powder had the following analysis in weight percent.

Carbon .012 Manganese .150 Phosphorus .010 Sulphur .017 Silicon .007 Nickel 1.760 Molybdenum .430 Hydrogen loss .170 Iron Balance The alloy steel powder had an apparent density of 3.01 grams/cc., a green density at a compaction pressure of 30 tons per square inch of 6.51 grams/cc. and a green strength using .75 zinc stearate lubricant, after pressing at 30 tons per square inch of 1160 p.s.i.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter of the invention.

I claim:

1. Finely divided atomized alloy, steel powder to be subsequently annealed and used in powder metallurgy processes, comprising a plurality of agglomerated alloy steel particles consisting essentially of by weight up to 0.40% carbon; an element selected from the group consisting of 0.20 to 3.0% nickel, 0.20 to 1.0% chromium, 0.20 to 1.0% molybdenum and mixtures thereof; less than 0.30% manganese, less than 0.10% silicon; and the balance iron; said particles after annealing having an apparent density greater than 2.6 grams/cc. and a green density at a compaction pressure of 30 tons per square inch greater than 6.4 grams/ cc. with lubricant.

2. The steel powder of claim 1, wherein said steel powder contains less than 0.05% of aluminum and less than 0.05% of titanium.

3. The steel powder of claim 1, wherein the carbon content is in the range of 0.06 to 0.12% by weight.

4. Finely divided atomized alloy steel powder to be subsequently annealed and used in powder metallurgy processes, comprising a plurality of agglomerated alloy steel particles consisting essentially of by weight from 0.06% to 0.12% carbon; an element selected from the group consisting of 0.20 to 3.0% nickel, 0.20 to 1.0% chromium, 0.20 to 1.0% molybdenum and mixtures thereof; from 0.05 to 0.50% manganese, from 0.01 to 0.10% silicon, and the balance iron, said particles after annealing having an apparent density greater than 2.6 grams/ cc. and a green density at a compaction pressure of 30 tons per square inch greater than 6.4 grams/cc. with lubricant.

5. Finely divided annealed alloy steel powder to be used in powder metallurgy processes, comprising a plurality of agglomerated alloy steel particles consisting essentially of by weight from 0.001% to 0.02% carbon; an element selected from the group consisting of 0.20 to 3.0% nickel, 0.20 to 1.0% chromium, 0.20 to 1.0% molybdenum and mixtures thereof; from 0.01% to 0.25% oxygen, from 0.05% to 0.50% manganese, from 0.01% to 0.10% silicon, and the balance iron, said annealed particles having an apparent density greater than 2.6 grams/ cc. and a green density at a compaction pressure of 30 tons per square inch greater than 6.4 grams/cc. with lubricant.

References Cited UNITED STATES PATENTS 3,528,081 9/1970 Huseby et al 0.5 BA 3,597,188 8/1971 Neumann 75-05 BA L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner 

